1. Field of the Invention
The present invention relates to the operation of liquid-continuous distillation columns, in particular bubble columns.
2. Related Information
In its broadest sense a bubble column may be a column of liquid with a stream of gas bubbles passing upward through the liquid. Thus, these systems (excluding the column) are comprised of at least two components, i.e., the liquid and the gas. In these systems, unless it is an esthetic display item such as a lava lamp, there is an interaction between the liquid and the gas. For example, in a fish tank, air bubbling up through a column of water is partially absorbed into the water. In other systems, for example, an impure gaseous mixture may be passed through a liquid wash to remove the impurities or to remove the desirable component into the liquid such as capturing volatile organic compounds (VOC) from air which is washed in countercurrent flow with a suitable VOC solvent.
The present invention is particularly useful to a more specialized form of bubble column wherein there are three phases, the liquid, the gas and solid. Very frequently the solid component is comprised of particles which may be inert or catalytic. The solid component may be internal structures in the column such as heat exchanger tubes, baffles, trays or plates. The column may be relatively open or may be a packed column. The packing may be inert or catalytic. The packing may be fluidized, slurried solids or may contain both slurry and packing structures. Thus, the bubble systems may be used for extraction and/or reaction and extraction.
Extractive distillation may be carried out to fractionate a mixture while a high boiling solvent for a material or impurities is introduced near the top of the column to selectively extract a material or impurity during the distillation. However, there is no liquid level in the column above the kettle, thus vapor continuous and not a bubble column.
Historically bubble columns are operated liquid full with vapor flow rates that promote gross back mixing. Internals are sometimes employed to help promote plug flow of the vapor phase but back mixing of the liquid phase causes conditions where little or no fractionation occurs. The normal recommended vapor velocities are significantly below 50% of the jet flood point. In fact, the term “flood point” is not usually used in bubble column operation and must be defined by ordinary distillation tower operation. A principal industrial use of bubble columns is in the Fischer-Tropsch synthesis.